Scientists used to think that fluoride could harden tooth enamel, helping it retain the minerals that protect teeth from the acid produced by our oral flora. But recent work has shown that fluoride doesn’t really penetrate past the tooth’s thinnest outer layer, suggesting that something other than hardening is going on.

To drill deeper into this toothsome mystery, researchers whipped up a set of artificial choppers, made of the same stuff as teeth. And they used atomic force microscopy to take a closer look at how bacteria interact with this dental material. They found that three different strains of cavity-causing bugs cling less tightly to enamel that’s been rinsed with fluoride.

The bacteria carry a net negative charge on their surfaces. So the negatively charged fluoride ions in the treated enamel may be literally repulsive to the bacteria. Which causes them to bite the dust.